1. Field of the Invention
This invention relates to in situ recovery of shale oil, and more particularly to techniques for explosively expanding formation toward horizontal free faces within a retort site for forming an in situ oil shale retort.
2. Description of the Prior Art
The presence of large deposits of oil shale in the semi-arid high plateau region of the Western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596, 4,043,597; 4,043,598; and 4,192,554, which are owned by the assignee of this application and incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the fragmented mass and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous material to product heat, combustion gas, and combusted oil shale. By continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort. The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
U.S. Pat. No. 4,192,554, for example, discloses a method for explosively expanding formation containing oil shale toward horizontal void spaces within a retort site to form a fragmented mass of formation particles in an in situ oil shale retort. The techniques disclosed in that patent are exemplary of a "horizontal void volume" or "horizontal free face" method of forming an in situ oil shale retort. In such a method, a plurality of vertically spaced-apart voids, each of which can have a horizontal cross section similar to that of the retort being formed, are excavated one above another within the retort site. A plurality of vertically spaced-apart zones of unfragmented formation are temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated, preferably in a single round, to explosively expand each unfragmented zone into the voids above and/or below it to form a fragmented mass having a void volume substantially equal to the void volume of the initial mined voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
When forming an in situ oil shale retort by explosively expanding formation toward horizontal free faces, rather large underground voids may have to be excavated. For example, in the retort system disclosed in U.S. Pat. No. 4,192,554, horizontal void levels within the retort site are approximately 160 feet wide and 160 feet long with pillars to stabilize the roof. Underground void levels of this size can result in large roof spans and the possibility of unsafe working conditions unless adequate precautions are taken. One or more roof-supporting pillars are typically left temporarily in place in each horizontally extending void to minimize the extent of open span. Such load-supporting pillars can provide stability in the horizontal voids where operating personnel are present so that safety hazards, such as dangeroius rock falls, can be avoided. These pillars are explosively expanded before explosive expansion of overlying and/or underlying oil shale formation toward the excavated horizontal void, when forming the fragmented mass.
When such an excavated horizontal void has a relatively large height, say on the order of 35 feet or more, for example, the roof-supporting interior pillar or pillars left in such a void can occupy a substantial portion of the horizontal cross sectional area of the retort. For example, in the retort system disclosed in U.S. Pat. No. 4,192,554, a large centrally located support pillar approximately 70 feet wide and 116 feet long is left in one horizontal void approximately 160 feet wide and 160 feet long. Such pillars are relatively large in horizontal cross section so that the height-to-width ratio of the pillar is not excessive, and structural failure of the pillar can be avoided.
When forming an in situ retort by explosive expansion of formation toward horizontal voids, explosive charges for expanding the zones of formation toward the voids can be placed in vertical blast holes drilled in the zones of formation adjacent the voids. The blast holes are commonly spaced apart horizontally from one another in an array that extends across the entire horizontal cross section of each void, with outer rows of blast holes being at or near the vertical side boundaries of the retort site. Explosive in the blast holes adjacent the side boundaries of the retort site is not as efficient in fragmenting the oil shale as explosive in blast holes nearer the center region of the retort site because of constraining effects of the adjacent walls of unfragmented formation at the side boundaries of the retort site.
Roof-supporting interior pillars in the horizontal voids can interfere with ease of access for placement of explosive charges in the more centrally located formation beneath such pillars. Consequently, long vertical blast holes have been drilled downwardly into the formation beneath such pillars, via access provided from within a horizontal void at the next higher working level, in order to reach the formation below the pillars. It would be desirable to minimize any such difficulty in placing explosive charges in formation within the central region of such a zone of unfragmented formation, since explosive placed in such a central region is more efficient in fragmenting the formation than explosive charges placed near the side boundaries of the retort site.